Analysis Of Evolution Characteristics Of Groundwater Quality Under River Infiltration Recharge With Reclaimed Water

Managed aquifer recharge（MAR）with reclaimed water may pose environment risks.YINWENJICHAO,a groundwater recharge project by reclaimed water in river infiltration method has been in operation for over 10 years.To investigate the actual impact on native groundwater under long-term MAR,this paper studies the evolution characteristics of groundwater quality through statistical calculations and hydrochemical simulations based on the reclaimed water（recharge water）and groundwater monitoring data.The findings provide guidance for preventing the environmental risks of groundwater from the MAR projects with river infiltration method.The main conclusions are as follows:（1）Due to the effect of recharge,the hydrochemical type of groundwater changed from HCO₃-Ca-Mg into HCO₃-Na,which was the same type of that of recharge water.Cl^-was used as a conservative tracer,and the mixing ratio of recharge water and groundwater was detected by a physical mixing model of mass balance.There was a good consistency between the variations of calculated concentration and measured concentration of the main ions in groundwater,which indicated that the mixing was dominant in the groundwater hydrochemical change under long-term MAR.River infiltration did not directly affect the water quality and quantity of confined aquifer.（2）The hydraulic travel time to the 30 m depth was determined to be about 6.5months by obtaining the best-fit linear cross correlation between the concentrations of Cl^-in recharge water and those in groundwater.Comparing the other monitoring well data in the MAR site,it was found that the mixing had a significant impact on the concept model.Therefore,in application of this model,the monitoring wells should be located downstream and within 50 m from the recharge site.（3）NO₂-N,NH₄-N,Al,Fe,and Mn were identified as the typical contaminants in the source and sink of MAR according to the current relevant standards.The high hydraulic load operation mode of this MAR site was beneficial to the attenuation of NO₃-N and NO₂-N in recharge water,and its attenuation rate of total nitrogen was84.8%.The attenuation factor（A_f）of NH₄-N changed from A_f>1 to A_f<1 when the concentration fluctuations of NH₄-N in recharge water exceeded 4 mg/L,which revealed the adsorption-desorption process of NH₄-N in soil.This mechanism led to the overall attenuation rate of NH₄-N was 26.3%.The natural high concentrations of Al,Fe,and Mn in shallow groundwater of the MAR site were reduced by the dilution of recharge water.The simulations of PHREEQC showed that the typical minerals containing these metal contaminants had a weak dissolution and precipitation in the study site under long-term MAR.（4）The quality of shallow groundwater was improved under MAR.The evaluation of groundwater quality indicators showed that the number of indicators in Class I and Class II increased,while the number of indicators in Class IV and Class V decreased.This study considered that MAR by river infiltration with reclaimed water that met current quality standard had low pollution risks of salt,nitrogen,Al,Fe and Mn to groundwater in the study area.